This is the UK guidance
. It claims to be based on Council Regulation (EEC) 2913/92 establishing the Customs Code, and Commission Regulation (EEC) 2454/93, which lays down the provision for its implementation so the rules in Greece ought to be the same. Paragraph 2.2 for road vehicles states: "6 months however, if you are a non EU student or non-EU worker on a temporary assignment see paragraph 3.5". Paragraph 3.5 extends the period: "Non-EU students and non EU workers on temporary assignments. If your normal place of residence is outside the EU and you temporarily import a vehicle for use during your studies or assignment, the vehicle can remain under relief in the EU for the period of your studies or until the end of your assignment, see also paragraph 3.3." To be really sure you need to read the whole document.
Generally it seems the requirements for non-EU registered vehicles are pretty much the same as EU registered. Bring it in for more than 6 months and you have to register it.
PS I actually know very little but my curiosity has no bounds. If someone asks a question and I don't know the answer I have an irresistible urge to find out. Probably conditioned by 50 years of research.